A transparent material is a material through which visible light passes. A transparent material has a unique benefit that you can see through the material. A material cannot acquire transparency a posteriori by coloring. An a priori transparent material has been studied and developed in various industrial and artistic fields.
Since a transparent material can be applied in a wide range of uses, the transparent material has been required to have any of various properties as well as an opaque material. The conventional transparent or semi-transparent materials however fail to meet a requirement for strength or toughness on an article such as shoes and tires in practice.
In addition, a so-called transparent material may actually have various transparency. In the case that a material is muddy or hazy, the material would be unsuitable for preparing a transparent and aesthetic material.
Patent Literature 1 discloses a transparent crosslinked rubber composition comprising ethylene/alpha-olefin/non-conjugated polyene copolymer rubber, ethylene/polar monomer copolymer, and silica.
Adding silica for strength of a material, however, has been known to impair the transparency of the material. A silica-containing material is sometimes called as transparent material, but indeed the material is clouded or yellowed. In addition, a silica-containing material requires a complicated process to disperse silica in order to improve its transparency.
A silicon-based material such as silicone resin and silica-containing resin generally needs oil as a softener to have flexibility. The more flexibility a silicon-based material has, the more oil is needed. Oil in a silicon-based material however leads to a “bleeding” over time, and its use has been actually restricted.
Patent Literature 2 discloses a transparent composition comprising a rubber polymer containing two components having different refractive indices, and wet silica. Indeed the composition has an insufficient transparency, and cannot overcome the defects of conventional art. The prior composition also has the defects of silica.
Patent Literatures 3 to 5 disclose crosslinked polybutadiene compositions. The conventional compositions are mere good in coloration, and cannot contribute to transparency.
Even though there has been many prior studies and demands in the filed, the skilled artisan has not developed yet a composition having both high transparency and rubber properties such as hardness and strength. The fact is an evidence of great difficulties to achieve the combination of the properties at higher level.